In automobile coating with a water-based base coating material, when a three-wet multiple layered coating film obtained by a three-wet coating method, which comprises the step of simultaneously heating and curing a uncured first base coating film, uncured second base coating film and uncured clear coating film formed on an automobile body having an electrodeposition coating film on the surface thereof is compared with a two-coat-one-bake multiple layered coating film obtained by a two-coat-one-bake coating method, which comprises the step of simultaneously heating and curing a uncured base coating film (hereinafter referred to as a “second base coating film”) and uncured clear coating film formed on an automobile body having an electrodeposition coating film and an intermediate coating film (hereinafter referred to as a “first base coating film”) in order on the surface thereof, it is necessary to thin the thickness of the first base coating film in the three-wet multiple layered coating film as compared with the thickness of the first base coating film in the two-coat-one-bake multiple layered coating film in order to secure the smoothness of the coating film. As the result, since the chipping resistance of the multiple layered coating film is reduced, it is problem that the multiple layered coating film is destroyed when a stepping stone collides with the coating film during while moving the automobile, a damage thereof is reached to the steel plate, and it cause the separation at the interface between the steel plate and electrodeposition coating film, or at the interface between the electrodeposition coating film and first base coating film.
A variety of methods for solving the problem of the reduction of the chipping resistance have been proposed.
For example, JP 2006-265535 A (Patent Document 1) discloses a water-based intermediate coating material comprising an acrylic emulsion resin produced from the specified unsaturated monomers, a hydroxyl group-containing resin, a curing agent and a color pigment having the specified solid content.
In addition, JP 2010-082554 A (Patent Document 2) discloses a process for forming a multiple layered coating film comprising the step of applying a chipping resistant coating material having the limited values of the physical properties of the coating film, intermediate coating material, base coating material and clear coating material in order to form a multiple layered coating film.
However, in the composition of Patent Document 1, since there is no linking group having a large cohesive force, such as urethane bond effective in the chipping resistance in the resin backbone, the chipping resistance in the intermediate coating film is insufficient. In addition, since the minimum melt viscosity is low in the three-wet coating method, and it is insufficient to control the mixture of layers between the intermediate coating material and the base coating material, it is problem thereby that the appearance of the three-wet coating film is poor as compared with the two-coat-one-bake coating film.
In addition, in the method of forming the coating film disclosed in Patent Document 2, since the intermediate coating film contains flat pigments, the impact relaxation when a stepping stone collides is dispersed in the horizontal direction of the coating film, and it is possible to suppress the separation at the interface between the steel plate and electrodeposition coating film, or at the interface between the electrodeposition coating film and intermediate coating film. However, it is problem that the area of the separation of the coating film in the horizontal direction of the intermediate coating film is increased, and the appearance of the automobile body is impaired.